Mostly, hydrocracking catalysts comprise a hydrogenating group deposited on an acid carrier. The hydrogenating group is either a noble metal such as palladium or platinum, as mentioned in U.S. Pat. Nos. 3,825,027, 3,890,247 and 4,120,825, or an association of sulfides of metals from group VI B (molybdenum or tungsten) and from group VIII (cobalt or nickel) of the periodic classification of elements, as mentioned for example in U.S. Pat. Nos. 3,132,089 and 3,159,568.
The acid carrier is generally a halogenated alumina, an amorphous silica-alumina or a Y type zeolite wherein at least 70% of the initial sodium has generally been replaced by protons or ions of rare-earth metals. The zeolite carriers are not formed of pure zeolite but, in fact, contain from 10 to 80% of zeolite admixed with an inert or a less active carrier such as alumina or silica-aluminas.
The zeolite catalysts are generally used in hydrocracking operations for a high gasoline production (C. MARCILLY, J. P. FRANCK Catalysis by Zeolites. Edt by IMELIK and al., Elsevier Scientific Publish. Comp., Amsterdam, 1980, 93-104; Iranian Petroleum Institute, Bulletin 66, 1977, 17.) As a matter of fact, their particular selectivity is not well adapted to the manufacture of middle distillates, such as for example as kerosene and gas oil. The selectivity to middle distillate a is much better with catalysts prepared from amorphous carriers of the silica-alumina type, which are much less acid than the zeolites.
The mixed silica-aluminas best adapted to a high production of middle distillates are those containing from 15% to 60% by weight of SiO.sub.2 in the oxide. (M: CHOUDHARY, e.a., Ind. Eng. Chem. P.R.D., vol. 14 (2), 1975, 74-83).
Various recent patents mention the use, for hydrocracking of heavy cuts, of bridged clays associated with a catalytic metal from groups I B to VIII of the periodic classification of elements, such for example as a noble metal, nickel, cobalt, tungsten or molybdenum. U.S. Pat. No. 4,176,090, for example, discloses the use for hydrocracking of a bridged clay containing, for example, palladium. In this patent, however, no precise statement is made concerning the cut.
U.S. Pat. No. 4,248,739 discloses for example the use of bridged clays, in particular for the manufacture of catalysts containing such metals as palladium, platinum, cobalt, molybdenum, nickel, tungsten and rare earths. The obtained catalysts are particularly useful in the reactions of hydrocarbons conversion such for example as hydrocracking. However this patent gives no precise detail concerning the cuts or the performance.
On the other hand, the use of fluorinated clays, which are not bridged, in various reactions involving hydrocarbon conversion, in the presence or absence of hydrogen, has been considered or mentioned in the literature: A.C.S. Vol. 18 1973, Division of Petroleum Chemistry preprints meeting of Dallas, Apr. 8-13 pages 33 to 51 Hattori H et al., and U.S. Pat. No. 3,855,147.
Moreover, the use in hydrocracking and hydroisomerization reactions of fluorinated clays, wherein a portion or the totality of the aluminum in octahedral position has been replaced by nickel or cobalt, has been mentioned in A.C.S. Vol. 20, 1975, Division of Petroleum Chemistry, preprints, meeting of Philadelphia, April 6-11, pages 522 to 534 and claimed in U.S. Pat. No. 3,966,642.
All the clays mentioned in the preceding references pertain to the category of smectites, a description of which is given hereinafter.